1. Field of the Invention
The present invention relates to protective coatings for components exposed to high temperatures, such as components of a gas turbine engine. More particularly, this invention is directed to a process for removing a ceramic coating and an underlying metallic coating that lie on a second metallic coating on the surface of a component without removing or damaging the second metallic coating.
2. Description of the Related Art
Components located in the hot gas path of a gas turbine engine (e.g., turbine buckets, nozzles and shrouds) are often thermally insulated with a ceramic layer in order to reduce their service temperatures, which allows the engine to operate more efficiently at higher temperatures. These coatings, often referred to as thermal barrier coatings (TBC), must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. Coating systems capable of satisfying these requirements may include a metallic bond coat that adheres the thermal-insulating ceramic layer to the component, forming what is termed a TBC system. Metal oxides, such as zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO) or other oxides, have been widely employed as the material for the thermal-insulating ceramic layer. The ceramic layer, or topcoat, is typically deposited by air plasma spraying (APS), low pressure plasma spraying (LPPS), or a physical vapor deposition (PVD) technique, such as electron beam physical vapor deposition (EBPVD) which yields a strain-tolerant columnar grain structure. Bond coats are typically formed of an oxidation-resistant diffusion coating such as a diffusion aluminide or platinum aluminide, or an oxidation-resistant alloy such as MCrAlY (where M is iron, cobalt and/or nickel). MCrAlY-type bond coats are termed overlay coatings, and are deposited by physical or chemical vapor deposition techniques or by thermal spraying, e.g., APS, LPPS and high velocity oxy-fuel (HVOF), which entails deposition of the bond coat from a metal powder.
Though significant advances have been made with coating materials and processes for producing both the environmentally-resistant bond coat and the thermal-insulating ceramic topcoat, circumstances can arise where one or more of the TBC layers must be replaced. For example, removal may be necessitated by damage during engine operation, or during component manufacturing to address such problems as coating defects, handling damage, and the need to repeat noncoating-related manufacturing operations. Abrasive techniques for removing thermal barrier coatings generally involve grit blasting, vapor honing and glass bead peening, each of which is a slow, labor-intensive process that erodes the ceramic layer and bond coat, as well as the substrate surface beneath the coating. Nonabrasive processes for removing ceramic coatings include autoclaving and high pressure waterjet, the latter of which is reported in commonly-assigned U.S. Pat. Nos. 5,558,922, 6,099,655, 6,544,346 and 6,210,488, as well as U.S. Pat. Nos. 5,167,721 and Re. 35,611 to McComas et al. The waterjet technique disclosed by McComas et al. is described as being capable of removing plasma sprayed and sintered coatings whose cohesive strength is significantly less than that of the substrate on which the coating is deposited. In reference to a ceramic coating adhered to a substrate with a bond coat, McComas et al. report that the waterjet pressure can be adjusted to remove the ceramic coating without bond coat damage, or remove the bond coat without substrate damage if pressures of not more than 60,000 psi (about 4000 bar) are used.
Notwithstanding the above, TBC and bond coats can be difficult to remove and repair. If specific layers of a TBC system cannot be selectively removed from a component without damaging the other layers or the component substrate surface, it may be necessary to scrap the component. This situation is exasperated with TBC systems that make use of coating materials that are stronger than those used in conventional TBC systems, or that comprise more than two coating layers of similar materials, such as where only one of multiple bond coat layers requires removal. One example of such a coating system is a TBC system developed by the assignee of the present invention to have a relatively high-strength, dense vertically cracked (DVC) plasma-sprayed ceramic topcoat and a metallic bond coat having at least two layers. According to commonly-assigned with U.S. Pat. No. 5,817,372, such a bond coat has an inner layer (nearer the substrate) that is denser than a second layer on which the topcoat is deposited.